Porous tin oxide films

ABSTRACT

Initial film layers prepared from tin(II) chloride spontaneously generate open cavities when the initial film layers are thermally cured to about 400° C. using a temperature ramp of 1° C./minute to 10° C./minute while exposed to air. The openings of the bowl-shaped cavities have characteristic dimensions whose lengths are in a range of 30 nm to 300 nm in the plane of the top surfaces of the cured film layers. The cured film layers comprise tin oxide and have utility in gas sensors, electrodes, photocells, and solar cells.

BACKGROUND

The present invention relates to porous tin oxide films, and morespecifically, to methods of forming tin oxide films of substantiallyuniform large pore size.

Porous tin(IV) oxide (SnO₂) can be useful in many applications rangingfrom electrochemistry to optics, including solar cells, window coatingsand gas sensors. Depending on the final application, the requirements inpore size, pore quantity, and pore morphology can be different.

SUMMARY

Accordingly, a method is disclosed, comprising:

forming an initial solution comprising i) a tin(II) salt, ii) an organicsolvent capable of dissolving water, and iii) water;

aging the initial solution with agitation at a solution temperature andfor a period of time sufficient to generate a film-forming solutionwhich is capable of forming a cured film layer having a top surfacecomprising independent open cavities;

depositing the film-forming solution on a substrate, thereby forming aninitial film layer disposed on the substrate, the initial film layerhaving a temperature T1 less than a maximum curing temperature T2; and

increasing the temperature of the initial film layer from T1 to T2 at arate effective in forming the open cavities spontaneously whilecontacting the initial film layer with an oxygen-containing atmosphere,thereby forming a layered structure comprising the cured film layer;wherein

the cured film layer comprises a tin oxide selected from the groupconsisting of tin(II) oxide (SnO), tin(IV) oxide (SnO₂), andcombinations thereof,

the open cavities have respective characteristic dimensions whoselengths are in a range of 30 nm to 300 nm at the top surface of thecured film layer, the characteristic dimensions having a mean length,and

80% to 100% of the characteristic dimensions of the open cavities arewithin 10% of the mean length.

Also disclosed is a layered structure formed by the above-describedmethod.

Further disclosed is a device comprising the above-described layeredstructure.

Also disclosed is a structure, comprising:

a film of an oxide given by the formula Sn_(x)Z_(1-x)O_(y), wherein Z isa metal other than Sn, x is between 0.75 and 1, and y is at least 1.75;and

a substrate that underlies and is in contact with the film;

wherein:

the film includes interconnected pores that extend from the top surfaceof the film, the pores having respective characteristic dimensions atthe top surface between 30 nm and 300 nm, and

at least 80% of the characteristic dimensions fall within 10% of themean of the characteristic dimensions.

Also disclosed is a method of forming the above-described structure,comprising:

providing a solution that includes a metal salt and an organic solvent,wherein water has been added to the metal salt and the organic solvent;

allowing the solution to age for more than 1 hour after the water hasbeen added; depositing the aged solution onto a substrate; and thermallycuring the deposited solution, thereby forming the structure.

Further disclosed is a method comprising using the above-describedstructure as part of a gas sensor.

Also disclosed is a method comprising using the above-describedstructure to catalyze a redox reaction.

The above-described and other features and advantages of the presentinvention will be appreciated and understood by those skilled in the artfrom the following detailed description, drawings, and appended claims.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIGS. 1A-1C are scanning electron micrographs of cross-sections of thefilms of Examples 1A-1C, respectively.

FIG. 2 is a scanning electron micrograph of a cross-section of the filmof Example 2.

FIG. 3 is a scanning electron micrograph of a cross-section of the filmof Example 3.

FIG. 4 is a scanning electron micrograph of a cross-section of the filmof Example 4.

FIG. 5 is a scanning electron micrograph of a cross-section of the filmof Example 5.

FIG. 6 is a scanning electron micrograph of a cross-section of the filmof Example 6.

FIG. 7 is a scanning electron micrograph of a cross-section of the filmof Example 7.

FIG. 8 is a scanning electron micrograph of a cross-section of the filmof Example 8.

FIG. 9 is a scanning electron micrograph of a cross-section of the filmof Example 9.

FIG. 10 is a scanning electron micrograph of a cross-section of the filmof Example 10.

FIG. 11 is a scanning electron micrograph of a cross-section of the filmof Example 11.

FIG. 12 is a scanning electron micrograph of a cross-section of the filmof Example 12.

FIG. 13 is a scanning electron micrograph of a cross-section of the filmof Example 13.

FIG. 14 is a scanning electron micrograph of a cross-section of the filmof Example 14.

FIG. 15 is a scanning electron micrograph of a cross-section of the filmof Example 15.

FIG. 16 is a scanning electron micrograph of a cross-section of the filmof Example 16.

FIG. 17 is a scanning electron micrograph of a cross-section of the filmof Example 17.

FIG. 18 is a scanning electron micrograph of a cross-section of the filmof Example 18.

FIG. 19 is a scanning electron micrograph of a cross-section of the filmof Example 19.

FIG. 20 is a scanning electron micrograph of a cross-section of the filmof Example 20.

FIG. 21 depicts a photomicrograph of the substrate containing theinterdigitated electrodes.

FIG. 22 is a graph and accompanying composite image showing the responseof the film of Example 10 (not having large pores) to various lowconcentrations of acetone.

FIG. 23 is a graph and accompanying composite image showing the responseof the film of Example 11 (having large pores) to various lowconcentrations of acetone.

FIG. 24 show the x-ray reflectivity (XRR) of the films of Examples 10(no large pores) and 11 (having large pores).

FIG. 25 shows the x-ray diffraction (XRD) scans of the film of Examples10 (no large pores) and 11 (having large pores).

FIGS. 26A-26C are cross-sectional layer diagrams showing a process ofmaking the disclosed film layer containing self-forming cavities.

FIG. 27 is a scanning electron micrograph of a cross-section of the filmof Example 21,

FIG. 28 is a scanning electron micrograph of a cross-section of the filmof Example 22.

DETAILED DESCRIPTION

Disclosed is a template-free, one step method for preparing a cured filmlayer starting from a tin(II) salt. The cured film layer (also referredto herein as “film layer” or “film”) is disposed on a top surface of asubstrate. The cured film layer comprises a metal oxide comprisingtin(II) and/or tin(IV), and optionally an auxiliary metal oxidecomprising a metal other than tin. Also disclosed are structures, moreparticularly layered structures, comprising the film layer disposed on asubstrate. The layered structures can be planar or non-planar. The filmlayers are preferably substantially planar and have utility ascomponents for gas sensors and other devices capable of utilizing acavity-containing inorganic film layer. For example, the film layers canfunction as catalysts for redox reactions in devices such as electrodes,photocells, and/or solar cells.

The mole percent (mol %) of total tin ions (i.e., tin(II) plus tin(IV))of the cured film layer is greater than 50 mol % and less than or equalto 100 mol % based on total moles of all metal ions of the film layer.More specifically, the cured film layer comprises an oxide given by theformula Sn_(x)Z_(1-x)O_(y), wherein Z is a metal other than Sn, x isbetween 0.75 and 1, and y is at least 1.75.

The cured film layer has regular bowl-shaped cavities that are open atthe top surface of the cured film layer. The cavities (also referred toherein as “interconnected pores”, “pores”, and “open cavities”) have amaximum depth preferably less than the thickness of film layer (i.e.,the cavities preferably do not extend to the bottom surface of the filmlayer in contact with the substrate). The opening of a given cavity atthe top surface of the film layer can have an elliptical, oval,circular, or irregular shape. A given cavity has a characteristicdimension in the plane of the top surface of the film layer. Thecharacteristic dimension can be a diameter of the smallest circlecapable of encompassing the opening of the cavity as measured at the topsurface of the film layer, or the characteristic dimension can be amajor axis of a non-circular shape (e.g., oval, ellipse). The lengths ofthe characteristic dimensions of the openings are between 30 nm and 300nm. The characteristic dimensions also have a narrow size distribution,wherein 80% to 100% of the characteristic dimensions are within 10% ofthe mean length of the characteristic dimensions.

The cavities can constitute about 5% to 15%, more particularly 8% to12%, or most particularly about 10% of the volume of the film layer. Inan embodiment, the cavities constitute at least 10% of the volume of thefilm layer.

The method utilizes a film-forming solution capable of undergoing asol-gel transition. In one embodiment, the film-forming solution is anaged solution of a tin(II) salt. Optionally, the film-forming solutioncan be prepared by combining the aged solution of the tin(II) salt withone or more separately prepared and aged solutions of auxiliary metalsalts. The auxiliary metal salts comprise metals other than tin. Itshould be understood that use of an auxiliary metal salt is optional.

The method excludes a templating sacrificial pore generator forgenerating the cavities of the cured film layer. A templatingsacrificial pore generator is defined herein as any material other thanthe tin(II) salt and any optional auxiliary metal salt that is consumedor degraded during the curing process and is suitable for generating thecavities of the cured film layer.

An initial solution is formed comprising the tin(II) salt, an organicsolvent capable of dissolving water, and water. The tin(II) salt ispreferably dissolved in the organic solvent before the addition ofwater. When an auxiliary metal salt is used, separate initial solutionsare also prepared for each auxiliary metal salt in an organic solventcapable of dissolving water, and water. Each of the initial solutions isindependent. The initial solutions can comprise the same organic solventor different organic solvents. When different organic solvents are used,the combined organic solvents of the film-forming solution should becapable of dissolving all of the metal salts of the film-formingsolution.

The metal salt concentration of a given initial solution is about 0.01to 1.0 g/mL based on total weight of the metal salt.

The tin(II) salt is preferably a tin(II) halide selected from the groupconsisting of tin(II) chloride, tin(II) bromide, tin(II) iodide, andcombinations thereof. In an embodiment, the tin(II) salt is tin(II)chloride. The initial solution can comprise a mixture of tin(II) salts.

An auxiliary metal salt preferably comprises an ion of a metal selectedfrom the group consisting of Pd, Zn, Ga, In, Pt, Ce, W, and Cu. Morespecific auxiliary metal salts include indium(III) salts. In anembodiment, the auxiliary metal salt is an indium(III) salt selectedfrom the group consisting of indium(III) chloride, indium(III) bromide,indium(III) iodide, indium nitrate, and combinations thereof.

When indium(III) is present, the film-forming solution can comprise atin(II):indium(III) molar ratio between 50:50 and 100:0, morespecifically between 80:20 and 100:0, and even more specifically between85:15 and 100:0.

The organic solvent preferably has a boiling point (BP) greater than 75°C. at one atmosphere pressure. Non-limiting organic solvents includealcohols (e.g., 2-methoxyethanol (BP 124° C.), 2-ethoxyethanol (BP 135°C.), 1-propanol (97° C.), iso-propyl alcohol (BP 82.6° C.), n-butanol(BP 117° C.), 1-pentanol (BP 138° C.), 1-hexanol (BP 157° C.),cyclohexanol (BP 162), phenol (BP 182° C.), p-cresol (BP 202° C.),glymes (e.g., 1,2-dimethoxyethane, 1,2-diethoxy ethane), and polyglymes(e.g., diethylene glycol dimethyl ether, diethylene glycol diethylether, triethylene glycol dimethyl ether). In an embodiment, the organicsolvent comprises an alcoholic solvent comprising an alcohol group. Apreferred organic solvent is 2-methoxyethanol.

The initial solution is prepared at a solution temperature below theboiling points of the organic solvent and water. Preferably, the initialsolution is formed at a solution temperature between 0° C. and 75° C.,more preferably between 0° C. and 50° C., and most preferably ambienttemperature (i.e., a temperature between 0° C. and 40° C.).

Herein, “room temperature” (RT) refers to a temperature of 18-24° C.(e.g., typical air-conditioned laboratory). In an embodiment, theinitial solution is prepared at room temperature and aged at roomtemperature.

After the metal salt of a given initial solution is dissolved, the wateris added in an amount of 1.0 to 4.0 molar equivalents based on totalmoles of metal salt of the given initial solution.

The initial solution is then aged by agitating the initial solution at asolution temperature and for a period of time sufficient to generate afilm-forming solution capable of forming the disclosed cured film layerhaving a top surface comprising independent open cavities. This solutiontemperature for the aging process is preferably below the boiling pointsof the organic solvent and water, more preferably between 0° C. and 75°C., even more preferably between 0° C. and 50° C., and most preferablyambient temperature. In an embodiment the aging process is performed atroom temperature. The period of time of the aging process is preferablygreater than 1 hour and less than 51 hours, and more preferably 4 hoursto 48 hours. In an embodiment, the initial solution is agitated at roomtemperature for a period of time greater than 1 hour and less than 51hours. In another embodiment, the initial solution is prepared at roomtemperature and aged at room temperature for a period of time greaterthan 1 hour and less than 51 hours. The resulting aged initial solutionderived from the tin(II) salt is a film-forming solution. It should beunderstood that the optimum aging conditions (time and solutiontemperature) for the initial solution can vary for different organicsolvents, metal salts, amount of water, and metal ion concentration.

When one or more auxiliary metal salts are used, desirable cavityformation is favored by separate preparation and aging of each auxiliarymetal salt solution. An auxiliary initial solution comprises i) anauxiliary metal salt comprising an ion of a metal other than tin, ii) anorganic solvent capable of dissolving water, and iii) water.

The auxiliary initial solution can be prepared at a solution temperaturebelow the boiling points of the organic solvent and water. Preferably,the auxiliary initial solution is formed at a solution temperaturebetween 0° C. and 75° C., more preferably between 0° C. and 50° C., andmost preferably ambient temperature (i.e., a temperature between 0° C.and 40° C.).

Preferably, the auxiliary initial solution is aged for more than 1 hourand less than 51 hours, and more preferably 4 hours to 48 hours. In anembodiment, the auxiliary initial solution is agitated at roomtemperature for more than 1 hour and less than 51 hours. In anotherembodiment, the auxiliary initial solution is prepared at roomtemperature and aged at room temperature for more than 1 hour and lessthan 51 hours. The resulting aged auxiliary initial solution is referredto as an auxiliary film-forming solution.

Adding the separately prepared and aged auxiliary film-formingsolution(s) to the film-forming solution derived from the tin(II) saltprovides a combined film-forming solution capable of forming a curedfilm layer having a top surface comprising independent open cavities.The combined film-forming solution can comprise one or more separatelyprepared and aged auxiliary film-forming solutions.

When deposited on a top surface of a substrate, the film-formingsolution (or the combined film forming solution) provides an initialfilm layer that is substantially or wholly free of cavities. Thermallycuring the initial film layer spontaneously generates a plurality ofsubstantially regularly-spaced bowl-shaped open cavities at the topsurface of the cured film layer.

The film-forming solution (or the combined film forming solution) can bedeposited on a substrate using any suitable technique (e.g., spincoating, spray coating, dip coating, slot coating, roll coating, and thelike) to produce the cavity-free initial film layer of uniformthickness. The thickness of the initial film layer can be between 1 and600 nm, more particularly between 30 and 200 nm.

The substrate can comprise one or more layers, which can compriseinorganic and/or organic materials such as metals, carbon, or polymers.Exemplary substrate materials include semiconducting materials such as,for example, Si, SiGe, SiGeC, SiC, Ge alloys, GaAs, InAs, InP, as wellas other III-V or II-VI compound semiconductors. The substrate can alsocomprise a layered semiconductor such as Si/SiGe, or asemiconductor-on-insulator (SOI). The substrate can contain aSi-containing semiconductor material (i.e., a semiconductor materialthat includes Si) and an insulator material (i.e., silicon dioxide,silicon nitride, and quartz). The semiconductor material can be doped,undoped or contain both doped and undoped regions therein.

For gas sensor applications, the top surface of the substrate comprisesa conductive patterned metal layer containing a zero-valent metaldisposed on an underlying insulating layer (e.g., metal oxide, organicinsulating material). The patterned metal layer is suitable as anelectrically conductive electrode. More specifically, the patternedmetal layer can comprise interdigitated conductive electrodes of Cr/Pthaving a comb-comb geometry as shown in the photomicrograph of FIG. 21.The patterned metal layer can be formed using any suitable technique(e.g., electron-beam lithography). The underlying insulating layer canbe an organic material (i.e., a polymer such as poly(methylmethacrylate), polystyrene and the like), an inorganic material (i.e.,silicon oxide), or an organometallic material (e.g., a silsesquioxanepolymer containing alkyl-silicon groups). The underlying insulatinglayer has a thickness of about 500 nm to 1.5 micrometers. For this gassensor example, the top surface of the substrate comprises the topsurface of the patterned metal layer and the top surface areas of theunderlying insulating layer having no patterned metal layer disposedthereon. Accordingly, the bottom surface of the initial film layer hascontact with the top surface of the patterned metal layer and the topsurface areas of the underlying insulating layer that have no metallayer disposed thereon.

The initial film layer has a temperature T1 that is less than a maximumcuring temperature T2. Preferably, T1 is between 0° C. and 75° C., morepreferably between 0° C. and 50° C., and most preferably, T1 is between0° C. and 40° C. (i.e., ambient temperature).

The initial film layer is cured by gradually increasing the temperatureof the initial film layer from T1 to a higher maximum cure temperatureT2, which is preferably between 300° C. and 800° C., while contactingthe initial film layer with an oxygen-containing atmosphere (e.g., air).The temperature of the initial film layer is increased at a rateeffective in forming open cavities whose openings extend from the topsurface of the cured film layer to a depth less than the thickness ofthe cured film layer. Preferably, the rate can be 1° C./minute to 10°C./minute while exposing the initial film layer to an oxygen-containingatmosphere. In an embodiment, T2 is between 300° C. and 500° C.

Herein, gradually increasing the temperature using a defined rate oftemperature increase is referred to as “ramping the temperature”. Agraphic depiction of the temperature dependence on time has a positiveslope and is referred to as a “temperature ramp”. Typically, thetemperature ramp is linear (i.e., a positive-sloped line between T1 toT2). The temperature ramp can be a non-linear (i.e., a positive-slopedcurve between T1 and T2 resulting from a changing rate between T1 andT2).

The curing process can comprise one or more temperature ramps separatedby respective optional hold times at respective intermediatetemperatures after the ramps. For a given hold time, the film layer isheated (baked) at a constant temperature (e.g., the maximum temperatureof the previous ramp) for a suitable period of time, generally about 1minute to about 50 hours.

As a non-limiting example using two temperature ramps, the curingprocess can comprise heating the initial film layer using a firsttemperature ramp from T1 to an intermediate temperature greater than T1,followed by a hold time greater than 1 minute at the intermediatetemperature, followed by a second temperature ramp from the intermediatetemperature to T2, wherein T2 is greater than the intermediatetemperature, and wherein the first temperature ramp and the secondtemperature ramp have independent rates, generally between 1° C./minuteand 10° C./minute. In an embodiment, T1 of the initial film layer isless than 75° C., the intermediate temperature is between 100° C. to300° C., and T2 is between 300° C. and 800° C. In another embodiment, T2is between 350° C. and 450° C. In another embodiment, the hold time atthe intermediate temperature is 1 minute to 10 hours. The first andsecond temperature ramps can have the same or different heating rates.

The cavities form spontaneously during the curing process, producing acured film layer having substantially greater surface area compared tothe initial film layer. Although the method utilizes tin(II) chloride asa starting material, the cured film layer can comprise tin substantiallyin the form of tin(IV) oxide. The cured film layer has a thickness lessthan 600 nm, more specifically between 1 nm and 600 nm.

FIGS. 26A-26C are cross-sectional layer diagrams illustrating theprocess of forming of a cured film layer comprising the cavities. Afilm-forming solution prepared as described above using an initialsolution of a tin(II) salt, organic solvent, and water is applied to atop surface 12 of a substrate 10 (FIG. 26A), thereby forming layeredstructure 20 (FIG. 26B). Layered structure 20 comprises initial filmlayer 22 disposed on top surface 24 of substrate 10. Initial film layer22 has thickness h′ and has a temperature T1 preferably between 0° C.and 75° C. Initial film-layer 22 is substantially free of cavities.Thermally curing initial film layer 22 using one or more temperatureramps and optional hold times after the ramps forms layered structure 30(FIG. 26C). The rate of increase of the temperature of a given ramp canbe 1° C. to 10° C. per minute. A hold time after the given ramp, whenused, can be 1 minute to 50 hours. Each successive ramp has a maximumtemperature greater than the maximum temperature of the previous ramp.The last ramp has a highest temperature of 300° C. to about 800° C.(i.e., the maximum cure temperature). Layered structure 30 comprisescured film layer 31 disposed on surface 34 of substrate 10. Cured filmlayer 31 has thickness h″, top surface 33, open cavities 32 (which areopen at top surface 33), and closed cavities 35 (which are not open attop surface 33). Open cavities 32 can have a maximum depth d′ which isgreater than 0 and less than h″. Cured film layer 31 comprise a tinoxide and optionally an auxiliary metal oxide. Cured film layer 31 issubstantially free of organic solvent and water.

The examples that follow illustrate the formation of the disclosed curedfilm layers and their utility in preparing a gas sensor.

EXAMPLES

Materials used in the following examples are listed in Table 1.

TABLE 1 ABBREVIATION DESCRIPTION SUPPLIER SnCl₂ Tin (II) chloride, MW189.60, Sigma-Aldrich anhydrous SnCl₄ Tin (IV) chloride pentahydrate,Sigma-Aldrich MW 350.50 Sn(Ac)₂ Tin (II) acetate, MW 236.80,Sigma-Aldrich anhydrous SnTB Tin (IV) tert-butoxide, MW Sigma-Aldrich411.16, anhydrous InCl₃ Indium(III) chloride, MW 221.18, Sigma-Aldrichanhydrous MOE 2-Methoxyethanol Sigma-Aldrich

Preparation of Porous Metal Oxide Films Example 1A

The following procedure is representative. A sol-gel solution wasprepared by dissolving tin(II) chloride (2.58 g, 13.6 mmol) in2-methoxyethanol (MOE, 30 mL) with stirring. After 1.5 hours, water(0.37 g, 20.6 mmol, 1.5 molar equivalents based on tin(II) chloride) wasadded. The resulting mixture was stirred at RT for 28 hours, and thenspin-coated on a 2 inch silicon wafer at 1500 revolutions per minute(rpm) for 30 seconds. The resulting film was heated on a hot-plate inair from RT to 170° C. at 7° C./minute, held at 170° C. for 1 hour,heated from 170° C. to 400° C. at 5° C./minute, and held at 400° C. for2 hours. FIG. 1A is a scanning electron micrograph of a cross-section ofthe film of Example 1.

Example 1B

The procedure as described in Example 1 was followed except the mixturewas held at RT for 0.08 hour before being spin-coated. FIG. 1B is ascanning electron micrograph of a cross-section of the film of Example3.

Example 1C

The procedure as described in Example 1 was followed except the mixturewas held at RT for 1 hour before being spin-coated. FIG. 1C is ascanning electron micrograph of a cross-section of the film of Example4.

Example 2

The procedure as described in Example 1 was followed except the film washeated on a hotplate that was set to 170° C., held at 170° C. for 1hour, heated from 170° C. to 400° C. at 5° C./minute, and held at 400°C. for 2 hours. FIG. 2 is a scanning electron micrograph of across-section of the film of Example 2.

Example 3

The procedure as described in Example 1 was followed except the mixturewas held at RT for 4 hours before being spin-coated. FIG. 3 is ascanning electron micrograph of a cross-section of the film of Example3.

Example 4

The procedure as described in Example 1 was followed except the mixturewas held at RT for 51 hours before being spin-coated. FIG. 4 is ascanning electron micrograph of a cross-section of the film of Example4.

Example 5

The procedure as described in Example 1 was followed except 5.16 g oftin(II) chloride was used instead of 2.58 g. FIG. 5 is a scanningelectron micrograph of a cross-section of the film of Example 5.

Example 6

The procedure as described in Example 1 was followed except 5.16 g oftin(II) chloride was used instead of 2.58 g, and the mixture was held atRT for 51 hours before being spin-coated. FIG. 6 is a scanning electronmicrograph of a cross-section of the film of Example 6.

Example 7

The procedure as described in Example 1 was followed except 4.77 g oftin(IV) chloride pentahydrate was used instead of 2.58 g of tin(II)chloride. FIG. 7 is a scanning electron micrograph of a cross-section ofthe film of Example 7.

Example 8

The procedure as described in Example 1 was followed except 3.22 g oftin(II) acetate was used instead of 2.58 g, and the mixture was held atRT for 24 hours before being spin-coated. FIG. 8 is a scanning electronmicrograph of a cross-section of the film of Example 8.

Example 9

The procedure as described in Example 1 was followed except 4.24 g oftin(IV) tert-butoxide was used instead of 2.58 g, and the mixture washeld at RT for 26 hours before being spin-coated. FIG. 9 is a scanningelectron micrograph of a cross-section of the film of Example 9.

Example 10

The procedure as described in Example 1 was followed except 5.16 g oftin(II) chloride was used instead of 2.58 g, the mixture was held at RTfor 48 hours before being spin-coated. The substrate was the “IDEplatform” comprising Cr/Pt (5/50 nm) interdigitated electrodes (IDE) ina comb-comb geometry. The electrodes were formed using e-beamlithography onto 50 mm×50 mm Si/SiOx wafers, where the oxide layer has athickness of about 1 micrometer. FIG. 21 depicts a photomicrograph ofthe substrate containing the interdigitated electrodes. The film washeated in air on a hotplate that was set to 170° C., held at 170° C. for1 h, heated from 170° C. to 400° C. at 5° C./minute, and held at 400° C.for 2 hours. FIG. 10 is a scanning electron micrograph of across-section of the film of Example 10. FIG. 22 shows the response ofthe film of Example 10 to various low concentrations of acetone. FIGS.24 and 25 show the x-ray reflectivity and x-ray diffraction scans of thefilm of Example 10, respectively.

Example 11

The procedure as described in Example 1 was followed except 5.16 g oftin(II) chloride was used instead of 2.58 g, the mixture was held at RTfor 48 hours before being spin-coated and the substrate used was an IDEplatform instead of a 2-inch silicon wafer. FIG. 11 is a scanningelectron micrograph of a cross-section of the film of Example 11. FIG.23 shows the response of the film of Example 11 to various lowconcentrations of acetone. FIGS. 24 and 25 show the x-ray reflectivityand x-ray diffraction scans of the film of Example 11, respectively.

Example 12

A sol-gel solution was prepared by dissolving indium(III) chloride (2.19g) in 2-methoxyethanol (MOE, 30 mL) at 60° C. with stirring. After 4.5hours, deionized water (0.37 g) was added at RT. The resulting mixturewas stirred at RT for 28 hours, and then spin-coated on a 2 inch siliconwafer at 1500 revolutions per minute (rpm) for 30 seconds. The resultingfilm was heated in air on a hot-plate from RT to 170° C. at 7°C./minute, held at 170° C. for 1 hour, heated from 170° C. to 400° C. at5° C./minute, and held at 400° C. for 2 hours. FIG. 12 is a scanningelectron micrograph of a cross-section of the film of Example 12.

Example 13

The procedure as described in Example 12 was followed except the mixturewas held at RT for 4 hours before being spin-coated. FIG. 13 is ascanning electron micrograph of a cross-section of the film of Example13.

Example 14

The procedure as described in Example 12 was followed except the mixturewas held at RT for 51 hours before being spin-coated. FIG. 14 is ascanning electron micrograph of a cross-section of the film of Example14.

Example 15

A first sol-gel solution (A) was prepared by dissolving tin(II) chloride(2.58 g) in 2-methoxyethanol (MOE, 30 mL) with stirring. After 1.5hours, deionized water (0.37 g) was added. A second sol-gel solution (B)was prepared by dissolving indium(III) chloride (2.19 g) in2-methoxyethanol (MOE, 30 mL) at 60° C. with stirring. After 4.5 hours,deionized water (0.37 g) was added at RT. After solutions A and B werestirred separately at RT for 28 hours, 9 mL of solution A and 1 mL ofsolution B were mixed in a separate vial (Sn:In molar ratio was 90:10),and subsequently spin-coated on a 2-inch silicon wafer at 1500revolutions per minute (rpm) for 30 seconds. The resulting film washeated in air on a hot-plate from RT to 170° C. at 7° C./minute, held at170° C. for 1 hour, heated from 170° C. to 400° C. at 5° C./minute, andheld at 400° C. for 2 hours. FIG. 15 is a scanning electron micrographof a cross-section of the film of Example 15.

Example 16

The procedure as described in Example 15 was followed except solutions Aand B were stirred separately at RT for 4 hours before 9 mL of solutionA and 1 mL of solution B were mixed (Sn:In molar ratio was 90:10). FIG.16 is a scanning electron micrograph of a cross-section of the film ofExample 16.

Example 17

The procedure as described in Example 15 was followed except solutions Aand B were stirred separately at RT for 51 hours before 9 mL of solutionA and 1 mL of solution B were mixed (Sn:In molar ratio was 90:10). FIG.17 is a scanning electron micrograph of a cross-section of the film ofExample 17.

Example 18

A first sol-gel solution (A) was prepared by dissolving tin(II) chloride(2.58 g) in 2-methoxyethanol (MOE, 30 mL) with stirring. After 1.5hours, deionized water (0.37 g) was added. A second sol-gel solution (B)was prepared by dissolving indium chloride (2.19 g) in 2-methoxyethanol(MOE, 30 mL) at 60° C. with stirring. After 4.5 hours, deionized water(0.37 g) was added at RT. After solutions A and B were stirredseparately at RT for 28 hours, 5 mL of solution A and 5 mL of solution Bwere mixed in a separate vial, and subsequently spin-coated on a 2-inchsilicon wafer at 1500 revolutions per minute (rpm) for 30 seconds. Theresulting film (Sn:In molar ratio was 50:50) was heated in air on ahot-plate from RT to 170° C. at 7° C./minute, held at 170° C. for 1hour, heated from 170° C. to 400° C. at 5° C./minute, and held at 400°C. for 2 hours. FIG. 18 is a scanning electron micrograph of across-section of the film of Example 18.

Example 19

The procedure as described in Example 18 was followed except solutions Aand B were stirred separately at RT for 4 hours before 5 mL of solutionA and 5 mL of solution B were mixed (Sn:In molar ratio was 50:50). FIG.19 is a scanning electron micrograph of a cross-section of the film ofExample 19.

Example 20

The procedure as described in Example 18 was followed except solutions Aand B were stirred separately at RT for 51 hours before 5 mL of solutionA and 5 mL of solution B were mixed (Sn:In molar ratio was 50:50). FIG.20 is a scanning electron micrograph of a cross-section of the film ofExample 20.

Example 21

Separate aging. The general procedure of Example 15 was followed except1 mL of solution A (tin(II) chloride) and 9 mL of solution B(indium(III) chloride) were combined after separately aging solution Aand solution B. The resulting film-forming solution (Sn:In molar ratio10:90) was applied to the substrate and cured according to theconditions of Example 15. FIG. 27 is a scanning electron micrograph of across-section of the film of Example 21.

Example 22

Aging together. An initial solution was prepared by dissolving tin(II)chloride (0.325 g, 1.7 mmol) and indium(III) chloride (2.462 g, 11.1mmol) in 2-methoxyethanol (MOE, 28 mL) with stirring. After 1.5 hours,water (0.47 g, 26.1 mmol, 1.5 molar equivalents based on tin(II)chloride plus 2.1 molar equivalents based on indium(III) chloride) wasadded (Sn:In molar ratio was 10:90). The resulting mixture was stirredat RT for 28 hours, and then spin-coated on a 2 inch silicon wafer at1500 revolutions per minute (rpm) for 30 seconds. The resulting film wascured by heating the film on a hot-plate in air from RT to 170° C. at 7°C./minute, held at 170° C. for 1 hour, heated from 170° C. to 400° C. at5° C./minute, and held at 400° C. for 2 hours. FIG. 28 is a scanningelectron micrograph of a cross-section of the film of Example 22.

Table 2 summarizes the materials and amounts used to prepare the metaloxide films of Examples 1A to 14, and 22. In Example 22, the metal saltswere combined in one solution and aged together.

TABLE 2 Hold time @ RT Hold time before after Organic water water MetalMetal Salt(s) Solvent added Water added Example salt(s) (g) Solvent (mL)(hours) (g) (hours)  1A SnCl₂ 2.58 MOE 30 1.5 0.37 28  1B SnCl₂ 2.58 MOE30 1.5 0.37 0.08  1C SnCl₂ 2.58 MOE 30 1.5 0.37 1  2 SnCl₂ 2.58 MOE 301.5 0.37 28  3 SnCl₂ 2.58 MOE 30 1.5 0.37 4  4 SnCl₂ 2.58 MOE 30 1.50.37 51  5 SnCl₂ 5.16 MOE 30 1.5 0.74 28  6 SnCl₂ 5.16 MOE 30 1.5 0.7451  7 SnCl₄ 4.77 MOE 30 1.5 0.37 28  8 Sn(Ac)₂ 3.22 MOE 30 1.5 0.37 24 9 SnTB 4.24 MOE 30 1.5 0.37 26 10 SnCl₂ 5.16 MOE 30 1.5 0.74 48 11SnCl₂ 5.16 MOE 30 1.5 0.74 48 12 InCl₃ 2.19 MOE 30 4.5 0.27 28 13 InCl₃2.19 MOE 30 4.5 0.27 4 14 InCl₃ 2.19 MOE 30 4.5 0.27 51 22 SnCl₂/InCl₃0.325/2.462 MOE 28 4.5 0.47 28

Table 3 summarizes the materials and amounts used to prepare thecomposite metal oxide films of Examples 15 to 21. In these examples,solutions of each metal salt were prepared and aged separately.

TABLE 3 Hold time of Tin Hold time based sol of Indium after based solTin Indium water after water Metal salt(s) based based added addedExample (molar ratio) sol sol (hours) (hours) Solution mix 15SnCl₂/InCl₃ Sol Sol 28 28 9 mL of Sol from 90/10 from from Ex. 2 + Ex. 2Ex. 12 1 mL of Sol from Ex. 12 16 SnCl₂/InCl₃ Sol Sol 4 4 9 mL of Solfrom 90/10 from from Ex. 3 + Ex. 3 Ex. 13 1 mL of Sol from Ex. 13 17SnCl₂/InCl₃ Sol Sol 51 51 9 mL of Sol from 90/10 from from Ex. 4 + Ex. 4Ex. 14 1 mL of Sol from Ex. 14 18 SnCl₂/InCl₃ Sol Sol 28 28 5 mL of Solfrom 50/50 from from Ex. 2 + Ex. 2 Ex. 12 5 mL of Sol from Ex. 12 19SnCl₂/InCl₃ Sol Sol 4 4 5 mL of Sol from 50/50 from from Ex. 3 + Ex. 3Ex. 13 5 mL of Sol from Ex. 13 20 SnCl₂/InCl₃ Sol Sol 51 51 5 mL of Solfrom 50/50 from from Ex. 4 + Ex. 4 Ex. 14 5 mL of Sol from Ex. 14 21SnCl₂/InCl₃ Sol Sol 28 28 1 mL of Sol from 10/90 from from Ex. 2 + Ex. 2Ex. 12 9 mL of Sol from Ex. 12

Table 4 summarizes the heat treatments given to the films of Examples 1Ato 22. RT is room temperature (18-24° C.).

TABLE 4 Hold time after Hold Hold Metal water Ramp 1 Time Ramp 2 time @Salt(s) added Temp 1 Rate Temp 2 @ Temp 2 Temp 2 Rate Temp 3 Temp 3Example (mole ratio) (hours) Substrate (° C.) (° C./min) (° C.) (hours)(° C.) (° C./min) (° C.) (hours)  1A SnCl₂ 28 Si wafer RT 7 170 1 170 5400 2  1B SnCl₂ 0.08 Si wafer RT 7 170 1 170 5 400 2  1C SnCl₂ 1 Siwafer RT 7 170 1 170 5 400 2  2 SnCl₂ 28 Si wafer 170 0 170 1 170 5 4002  3 SnCl₂ 4 Si wafer RT 7 170 1 170 5 400 2  4 SnCl₂ 51 Si wafer RT 7170 1 170 5 400 2  5 SnCl₂ 4 Si wafer RT 7 170 1 170 5 400 2  6 SnCl₂ 28Si wafer RT 7 170 1 170 5 400 2  7 SnCl₄ 28 Si wafer RT 7 170 1 170 5400 2  8 Sn(Ac)₂ 24 Si wafer RT 7 170 1 170 5 400 2  9 SnTB 26 Si waferRT 7 170 1 170 5 400 2 10 SnCl₂ 48 IDE 170 0 170 1 170 5 400 2 platform11 SnCl₂ 48 IDE RT 7 170 1 170 5 400 2 platform 12 InCl₃ 28 Si wafer RT7 170 1 170 5 400 2 13 InCl₃ 4 Si wafer RT 7 170 1 170 5 400 2 14 InCl₃51 Si wafer RT 7 170 1 170 5 400 2 15 SnCl₂/InCl₃ 28 Si wafer RT 7 170 1170 5 400 2 (90/10) 16 SnCl₂/InCl₃ 4 Si wafer RT 7 170 1 170 5 400 2(90/10) 17 SnCl₂/InCl₃ 51 Si wafer RT 7 170 1 170 5 400 2 (90/10) 18SnCl₂/InCl₃ 28 Si wafer RT 7 170 1 170 5 400 2 (50/50) 19 SnCl₂/InCl₃ 4Si wafer RT 7 170 1 170 5 400 2 (50/50) 20 SnCl₂/InCl₃ 51 Si wafer RT 7170 1 170 5 400 2 (50/50) 21 SnCl₂/InCl₃ 28 Si wafer RT 7 170 1 170 5400 2 (10/90) 22 SnCl₂/InCl₃ 28 Si wafer RT 7 170 1 170 5 400 2 (10/90)

Examples 2 and 10 did not utilize a controlled temperature ramp. Forthese examples the coated substrate prepared at room temperature wasplaced directly on a pre-heated hotplate at 170° C. FIGS. 2 and 10(below) indicate the desired cavity formation is favored by utilizing atemperature ramp having a controlled slower rate of temperatureincrease.

Table 5 summarizes the properties of the films of Examples 1A to 22. Themost desirable films are those having “Yes” answers in each of thecolumns labeled “Presence of meso/macropores (Y/N)”, “Pores ellipticallyshaped (Y/N)”, “Pores regularly spaced (Y/N)”, and “Low polydispersityin pore major axis at surface (Y/N)”.

TABLE 5 Hold time Low after Presence of Pores Pores polydispersity watermeso/ elliptically regularly in pore major Film Metal added macroporesshaped spaced axis at surface thickness Example salt (hours) (Y/N) (Y/N)(Y/N) (Y/N) (nm) FIG. #  1A SnCl₂ 28 Yes Yes Yes Yes 59  1A  1B SnCl₂0.08 Yes Yes No No 55  1B  1C SnCl₂ 1 Yes Yes No No 55  1C  2 SnCl₂ 28 —— — — 50  2  3 SnCl₂ 4 Yes Yes Yes No 52  3  4 SnCl₂ 51 Yes Yes No No 52 4  5 SnCl₂ 4 Yes No No No 122  5  6 SnCl₂ 28 Yes Yes Yes Yes 132  6  7SnCl₄ 28 — — — — 47  7  8 Sn(Ac)₂ 24 — — — — 47  8  9 SnTB 26 — — — — 36 9 10 SnCl₂ 48 — — — — 125 10, 22, 24, 25 11 SnCl₂ 48 Yes Yes Yes Yes130 11, 23, 24, 25 12 InCl₃ 28 — — — — — 12 13 InCl₃ 4 — — — — — 13 14InCl₃ 51 — — — — — 14 15 SnCl₂/InCl₃ 28 Yes Yes Yes Yes 59 15 90/10 16SnCl₂/InCl₃ 4 Yes Yes Yes Yes 54 16 90/10 17 SnCl₂/InCl₃ 51 — — — — 4817 90/10 18 SnCl₂/InCl₃ 28 — — — — 35 18 50/50 19 SnCl₂/InCl₃ 4 — — — —40 19 50/50 20 SnCl₂/InCl₃ 51 — — — — 42 20 50/50 21 SnCl₂/InCl₃ 28 — —— — — 27 10/90 22 SnCl₂/InCl₃ 28 — — — — 39 28 10/90 (aged together)

Examples 1A-1C and 4 (FIGS. 1A-1C and 4, respectively) demonstrate thatdesirable cavities are favored by a hold time after addition of watergreater than 1 hour and less than 51 hours when the initial solution isprepared at room temperature.

Table 6 summarizes the elemental composition (as mole percent of eachelement) of the films of Examples 1A to 22 as determined by RutherfordBackscattering Spectrometry. Example 1A illustrates that films formedwith stannous chloride are essentially composed of 2 oxygens per tinatom (i.e., stannic oxide) after the heat treatment.

TABLE 6 Example Sn (%) In (%) O(%) Cl(%)  1A 31 ± 3 — 68 ± 3 1 ± 0.2  1BNot available  1C Not available  2 Not available  3 32 ± 3 — 67 ± 3 1 ±0.2  4 33 ± 3 — 66 ± 3 1 ± 0.2  5 33 ± 3 — 67 ± 3 —  6 33 ± 3 — 67 ± 3 — 7 Not available  8 Not available  9 Not available 10 32 ± 5 — 67 ± 5 1± 0.5 11 32 ± 5 — 67 ± 5 1 ± 0.5 12 — 37 ± 3 63 ± 3 — 13 — 41 ± 3 59 ± 3— 14 Not available 15 Not available 16 26 ± 5  7 ± 5 66 ± 3 1 ± 0.5 17Not available 18 Not available 19 17 ± 5 16 ± 5 64 ± 3 4 ± 0.5 20 Notavailable 21  6 ± 5 29 ± 5 64 ± 3 1 ± 0.5 22  8 ± 5 29 ± 5 61 ± 3 2 ±0.5

Gas Sensing Results

Example 10 (having negligible large cavities) and Example 11 (havinglarge cavities) were tested for gas sensing capability using acetone.The presence of the cavities was confirmed by x-ray reflectivity (XRR)analysis. The XRR scans of FIG. 24 clearly show a shift in criticalangle towards lower values for Example 11, indicating that the film ofExample 11 (FIG. 11) has a lower density (4.1 g/cm³) compared to thefilm of Example 10 (FIG. 10), density 4.95 g/cm³.

As shown in FIGS. 22 and 23, for a given acetone concentration theresponse of the film with large cavities (Example 11, FIG. 22) was atleast 2 times that of the film with negligible large cavities (Example10, FIG. 23). The films were of similar thickness and had the samecrystal structure and grain size, as indicated by the similar x-raydiffraction (XRD) scans of the two samples (FIG. 25).

The results indicate that the significant difference in the respectivegas sensing response to acetone of the films of Examples 10 and 11 wassolely due the differences in porous morphology (i.e., the presence ofcavities).

The Effect of Metal Composition

Examples 1A, 15, 18, 21, and 12 contain Sn/In molar ratios of 100:0,90:10, 50:50, 10:90 and 0:100, respectively, and each was aged 28 hours.The metal salt solutions of Example 15, 18, and 21 were separately aged.Comparing the SEMs of these films (FIGS. 1A, 15, 18, 27, and 12,respectively) shows that the tin-only film (Example 1A) containedrelatively uniform large cavities, whereas the indium-only film (Example12) appeared as a random arrangement of non-uniformly sized largecrystals having pyramidal and cubic shapes. The coatings containingcombinations of both metal oxides showed a progressive change from arelatively uniform film layer containing cavities to a non-uniform layerof random large crystals as the Sn/In molar ratio was changed from 90:10to 10:90. Example 18, which contained a 50:50 Sn/In molar ratio appearedamorphous, lacking both cavities and large crystals. Cavity formation isfavored by a film composition comprising more than 50 mol % tin based ontotal moles of metal of the film.

Effect of Aging Metal Salt Solutions Separately

The films of Example 21 (FIG. 27) and Example 22 (FIG. 28) have a Sn/Inmolar ratio of 10:90, and differ only by the aging process. In Example21, the metal salt solutions were prepared and aged separately, whereasin Example 22 the metal salts were combined in solution and agedtogether. Large crystal structures were obtained in Example 21 (FIG. 27,separate aging) corresponding to the crystals obtained in Example 12 fora Sn/In molar ratio of 0:100. The large crystals of Example 21 weremostly absent in the film of Example 22 (FIG. 28, metal salts agedtogether). The results indicate that separate aging of the metal saltsallows the dominant metal salt to have greater influence on theresulting film morphology. Therefore, separate aging is preferred as ameans of controlling size and uniformity of cavities when the tin(II)salt is the dominant metal salt.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof. When a range is used to express apossible value using two numerical limits X and Y (e.g., a concentrationof X ppm to Y ppm), unless otherwise stated the value can be X, Y, orany number between X and Y.

The description of the present invention has been presented for purposesof illustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the invention. Theembodiments were chosen and described in order to best explain theprinciples of the invention and their practical application, and toenable others of ordinary skill in the art to understand the invention.

What is claimed is:
 1. A layered structure comprising: a cured filmlayer disposed on a substrate, the cured film layer comprising i) a tinoxide selected from the group consisting of tin(II) oxide (SnO), tin(IV)oxide (SnO₂), and combinations thereof and ii) a top surface comprisingindependent open cavities, wherein the open cavities have respectivecharacteristic dimensions whose lengths are in the range of 30 nm to 300nm at the top surface of the cured film layer, the characteristicdimensions having a mean length, and 80% to 100% of the characteristicdimensions of the open cavities are within 10% of the mean length. 2.The layered structure of claim 1, wherein the tin oxide of the filmlayer is substantially tin(II) oxide.
 3. The layered structure of claim1, wherein the tin oxide of the film layer is substantially tin(IV)oxide.
 4. The layered structure of claim 1, wherein the top surface ofthe substrate comprises a pattern, the pattern comprising a zero-valentelectrically conductive metal, and the cured film layer has contact withthe pattern.
 5. The layered structure of claim 4, wherein the patterncomprises interdigitated electrodes.
 6. The layered structure of claim4, wherein the layered structure is suitable as a component of a deviceselected from the group consisting of gas sensors, electrodes,photocells, and solar cells.
 7. The layered structure of claim 1,wherein the cured film layer has a thickness between 1 nm and 600 nm. 8.The layered structure of claim 1, wherein the openings constitute atleast 10% of the volume of the cured film layer.
 9. The layeredstructure of claim 1, wherein the cured film layer comprises anauxiliary ion of a metal selected from the group consisting of Pd, Zn,Ga, In, Pt, Ce, W, Cu, and combinations thereof.
 10. The layeredstructure of claim 9, wherein the auxiliary ion is indium(III).
 11. Thelayered structure of claim 9, wherein the cured film layer comprises atin:indium molar ratio between 50:50 and 100:0 based on total moles oftin and indium present in the cured film layer.
 12. A device comprisingthe layered structure of claim
 1. 13. The device of claim 12, whereinthe device is selected from the group consisting of gas sensors,electrodes, photocells, and solar cells.
 14. A structure, comprising: afilm of an oxide given by the formula Sn_(x)Z_(1-x)O_(y), wherein Z is ametal other than Sn, x is between 0.75 and 1, and y is at least 1.75;and a substrate that underlies and is in contact with the film; wherein:the film includes interconnected pores that extend from the top surfaceof the film, the pores having respective characteristic dimensions atthe top surface between 30 nm and 300 nm, and at least 80% of thecharacteristic dimensions fall within 10% of the mean of thecharacteristic dimensions.
 15. The structure of claim 14, wherein thetop surface of the substrate comprises a patterned conductive metallayer disposed on an insulating layer.
 16. The structure of claim 14,wherein the film is substantially planar.
 17. The structure of claim 14,wherein said pores constitute at least 10% of the film volume.
 18. Thestructure of claim 14, wherein the thickness of the film is between 1 nmand 600 nm.
 19. The structure of claim 14, wherein Z is selected fromthe group consisting of Pd, Zn, Ga, In, Pt, Ce, W, and Cu.
 20. Thestructure of claim 19, wherein Z is In.